Transformer oil: primary properties

Transformer oil is a mineral oil product widely used in power industry. It is used in transformers, reactive components and oil-filled switches. Transformer oil isolates energized parts, dissipates heat and protects solid insulation from moisture.

The insulation characteristics of this material are well described by the so called dielectric strength. It depends on the presence of water and solid particles in the oil. Ideally, the oil should contain none of those.

To keep the insulation fluid flowing in cold temperature, the pour point must not exceed -45ºС. Good heat dissipation is only possible with low viscosity.

One of the most important parameters of the dielectric fluid is its oxidation stability. In the process of operation or handling, the oil is subject to many adverse factors, such as oxygen, sunlight, heat etc. The result is the oxidation of the oil, degrading its performance. To slow down the oxidation, special anti-oxidation additives are used with the oil.

Only fresh unused oils should be used in new transformers. Each batch of transformer oil must have a quality certificate form the manufacturer.

Before filling fresh dielectric fluid into the transformer, the oil must be purified from particulate matter, moisture and gas. Some of the methods available are centrifuging, adsorption and vacuum.

Centrifuging is the process of removing moisture and suspended solid particles by the influence of centrifugal force. It should be noted that this method only works for emulsified water and the particles with weight exceeding that of the oil. Centrifuge treatment is usually a preliminary purification step, or used only when filling transformers of 35kV or below. It is not recommended to keep the oil in a centrifuge for too long, since this may lead to oxidation due to the loss of the anti-oxidation additive in the oil.

Filtration is the process of passing the oil through porous barriers which capture contamination.

Adsorption is the process of purifying the oil from water and other contaminants by using special substances. These adsorbents can be natural or artificial. Most widely used sorbents today are zeolite, bleaching clays fuller’s earth etc.

Vacuum processing involves heating the oil to 50-60ºC and spraying it in the degassing chamber. An important component of the vacuum degassers are rings inside the chamber, which facilitate the distribution of oil in a thin film. Degassers may be equipped with two vacuum pumps, one facilitating regular removal of water vapor and gas, and the other creating high vacuum for extended degassing and oil drying.